Horde error : 500 No response from subprocess with exit signal: 0

You need to check if the server is having custom mysql version? like below?

root@server2 [~]# mysql --version
mysql Ver 14.16 Distrib 5.2.12-MariaDB, for unknown-linux-gnu (x86_64) using readline 5.1
This custom version of MySQL conflicts with the internal cPanel PHP, so I disabled the "usecpphp" file so that Horde could send email properly:
root@server2 [~]# 

in that case you need to do

root@server2 [~]# mv /var/cpanel/usecpphp /var/cpanel/usecpphp-disabled 

500 No response from subprocess with exit signal: 0

If you get the error "500 No response from subprocess with exit signal: 0" in horde while sending new emails:

Resolution :

You need to check if there is any custom mysql version is installed?, if yes then
mv /var/cpanel/usecpphp /var/cpanel/usecpphp-disabled
Your should be able to send emails now :)